I believe Simon pulled out the day before, by text at 6.54pm. Erin replied a few minutes later with "I've spent many hours this week preparing lunch or (?for) tomorrow". There was no mention of having constructed the beef wellingtons at that point. I don't think we know at what point they were put together, or the mushroom paste made, or the pastry closed over sealing both the pasties and the fate of the 4 who ate the tainted ones.
To me, it's quite plausible that they were constructed after this conversation and she did not make one specifically for Simon. Maybe she spread the tainted mushrooms across 4 pasties instead of 5, making them more potent and ensuring she had no death cap powder left over.
I agree, she probably left constructing them to the next day, shortly before the lunch. I seem to remember someone on the true crime group suggesting that even if the ingredients were prepared earlier, that the wellingtons should be put together last to avoid the pastry going soggy, which makes sense. They only take around 45 minutes in the oven to cook.
Perhaps she had a spare clean one unsealed that she could sprinkle the mushroom powder on last minute if Simon turned up.
It was mentioned both by the mushroom expert and by Erin herself that the dried (deathcap) mushrooms would have a strong smell, which is why I think it makes sense that they were powdered and added over the duxelles before sealing each one, rather than to another dish where the smell might be detected.
Alternatively she may have just rehydrated the chopped deathcaps to make a separate batch of duxelles. Red bowl for poison duxelles, white bowl for clean duxelles, or some such.
We don't which, but the powdered theory came about because she'd posted in the crime group about adding dehydrated powdered mushrooms to everything, which I gather was experimentation on her part in the lead up to the main event.
IMO